Conventional methods for collecting a sample of blood from a patient for laboratory analysis, such as blood gas analysis, include the use of hypodermic syringes of conventional size (1-5 cc) and capillary tubes which collect a much smaller blood sample. Regardless of the size of sample which is collected, there are certain precautions which must be observed in order to avoid erroneous analytical results, i.e., an anticoagulant must be present in the sampling device to prevent coagulation of the blood sample, and any air bubbles within the sample must be eliminated to avoid a loss of pCO.sub.2.
Before taking a blood sample with a syringe of conventional size and construction, the dead space between the plunger of the syringe and the needle is typically filled with an anticoagulant solution, such as a heparin solution, to prevent entrapment of air bubbles as well as to prevent coagulation of the sample. The volume of the heparin solution, however, may be substantial relative to the volume of the blood sample, thus leading to dilution thereof and consequent reduction in pCO.sub.2 values. In addition, the relatively large size of the blood sample withdrawn by the syringe may be disadvantageous when repeated samplings are necessary, or when dealing with infants.
With the availability of blood gas analyzers operable with substantially smaller blood samples (40-200 .mu.l), microsampler devices have become available. Such devices consist essentially of a hypodermic needle attached to a capillary tube which employ no plunger as in a conventional syringe and which are pre-loaded with a solid anticoagulant, typically a salt of heparin, thereby eliminating dilution problems. Such sampling devices are filled by the arterial or veinous pressure of the patient and are therefore less likely to entrap bubbles of air. Because of the restricted cross-sectional area of the capillary tube, however, dissolution and mixing of the anti-coagulant in the blood cannot be readily accomplished by shaking the device. Mixing of the heparin with the blood sample has been accomplished by the use of mechanical agitators, e.g., a spherical steel ball enclosed within the tube or an elongated magnetic "flea" which is moved along the length of the capillary tube by means of an external magnet. The use of such mechanical agitators, however, may cause physical damage to the cells of the blood sample.